The use of solenoids for providing mechanical force over a limited stroke is well known. They are used in countless applications throughout industry. Current is fed through an annular coil, and a plunger formed from ferromagnetic material is pulled into the coil when energised. It is however in the very nature of the device that as the plunger is pulled in, and reaches its point of rest, that the force experienced diminishes to zero, assuming a symmetrical disposition of coil and plunger. For many applications this is unsatisfactory, especially where a consistent force is required throughout the stroke irrespective of the physical displacement of the plunger.
Numerous design variations have been formulated to mitigate the effect of the inverse square law governing in whole or in part this effect. However, any attempt at linearisation of the solenoid's characteristics (force versus displacement) necessitates compromises in terms of electromagnetic simplicity of construction, (for example, the use of conical pole pieces and the like). Furthermore, the thrust experienced by the plunger is of course limited to the field produced by the coil.
To achieve higher forces therefore requires higher currents, resulting in higher heat loss. In fact, because heat loss is equal to the square of the current multiplied by the resistance of the coil, therefore a doubling in force requires a quadrapuling of heat dissipation capability.